Nuclear power station primary pump

A nuclear power station primary pump comprising:

• firstly a drive shaft for rotating a wheel and, above said wheel and concentrically about said shaft a heat exchanger forming a thermal barrier, a thermal barrier cover comprising a peripheral skirt provided at its base with a circular end wall extending beneath the heat exchanger, and a diffuser comprising a peripheral skirt provided at its base with a rim extending beneath the circular end wall of the thermal barrier cover, and secondly a body having fixed thereto the heat exchanger and the peripheral skirt of the thermal barrier cover, wherein the circular end wall of the thermal barrier cover co-operates with the base of the heat exchanger to define a bottom chamber for cold water and includes a central bore provided with a succession of circumferential and superposed rings forming a lip seal of the labyrinth type, and wherein the circular end wall cooperates both with the peripheral skirt of the thermal barrier cover and with the rim of the peripheral skirt of the diffuser to define clearance forming a space for maintaining stagnant cooling fluid.